Following loss of functional small bowel surface area, the intestinal epithelium mounts a remarkable adaptive response, with enhanced crypt cell proliferation and epithelial cell migration, increased villus height, crypt depth and nutrient absorption. This precisely balanced process of proliferation and differentiation is established and maintained by interactions between the epithelium and mesenchymal components such as intestinal myofibroblasts that surround the crypt. Studies proposed in this renewal will continue to use rodent models of intestinal adaptation following small bowel resection to study fundamental mechanisms of gut epithelial cell proliferation and differentiation. During the current project period, two genes that are markedly regulated during the early phase of the intestinal adaptive response, PC4/TIS7 and epimorphin, were shown to have profound effects on morphogenesis and differentiation of the epithelium. The major hypotheses of the current proposal are: 1) PC4/TIS7 plays a key role in growth regulation and terminal differentiation in gut epithelial cells, 2) Epimorphin produced by intestinal myofibroblasts regulates the formation and maintenance of the crypt-villus axis. To address these hypotheses, the Specific Aims are: 1) Continue an analysis of PC4/TIS7 function and regulation in intestinal epithelial cells, 2) Determine the in vivo function of PC4/TIS7, using transgenic mice, 3) Determine the mechanism(s) by which epimorphin/syntaxin 2 induces crypt-villus morphogenesis and cytodifferentiation, 4) Determine the in vivo function of epimorphin. These studies are significant because they will enhance our understanding the molecular regulation of the adaptive response and this knowledge will facilitate designing specific clinical regimens for short bowel syndrome and for diseases such as Crohn's disease, that are characterized by intestinal epithelial injury.